Dipalladium Complexes Research Articles

Mixed Thioether/Amine‐Functionalized Expanded Calixarenes, Related Macropolycycles, and Metal Complexes thereof

AbstractA series of tetraaza‐expanded calix[4]arene analogues (2–4), in which two of the four aryl groups are ethylene‐protected thiophenol components were prepared from tetraaldehyde 1. Two of these strapped macrocycles were characterized crystallographically, as well as the dipalladium complex of 4. The same tetraaldehyde had been previously used for the synthesis of a macrotricycle (5) and a macropentacycle (6), the coordination properties of which have been investigated. In particular, ligand 6 was shown to be able to encapsulate a Cu+ or a Ag+ ion in the tetrahedral coordination pocket offered by a bridgehead nitrogen atom and the three proximal thioether sulfur atoms.

Unsymmetrical Imidazopyrimidine-Based Ligand and Bimetallic Complexes.

With bimetallic catalysts becoming increasingly important, the development of electronically and structurally diverse binucleating ligands is desired. This work describes the synthesis of unsymmetric ligand 2,7-di(pyridin-2-yl)imidazo[1,2-a]pyrimidine (dpip) that is achieved in four steps on a multigram scale in an overall 54% yield. The ability of dpip to act as a scaffold for the formation of bimetallic complexes is demonstrated with the one-step syntheses of the dicopper complex [Cu2(dpip)(μ-OH)(CF3COO)3] (4), the dipalladium complex [Pd2(dpip)(μ-OH)(CF3COO)2](CF3COO)·CF3COOH (5), and the dimeric dinickel complex [Ni4(dpip)2(μ-Cl)4Cl2MeOH6][2Cl] (6) in good yields (79-92%). All bimetallic complexes were characterized by spectroscopic methods and X-ray crystallography, which revealed metal-metal distances between 3.4821(9) and 4.106(2) Å. Additionally, quantum chemical calculations were conducted on complex 4 and an analogous 1,8-naphthyridine-based dicopper complex to investigate the differences between the imidazopyrimidine motif reported here and the widely used 1,8-naphthyridine motif. Natural bonding orbital (NBO) and Mayer bond order (MBO) analyses validated the ability of dpip to coordinate metals more strongly. Finally, NBO calculations quantified the differences in the binding energy between the two pockets of the unsymmetrical dpip ligand.

Front Cover: Syntheses, Molecular Structures, and Countercation‐Induced Structural Transformation of Monomeric α‐Keggin‐Type Polyoxotungstate‐Coordinated Mono‐ and Dipalladium(II) Complexes (Eur. J. Inorg. Chem. 19/2021)

Front Cover: Syntheses, Molecular Structures, and Countercation‐Induced Structural Transformation of Monomeric α‐Keggin‐Type Polyoxotungstate‐Coordinated Mono‐ and Dipalladium(II) Complexes (Eur. J. Inorg. Chem. 19/2021)

Syntheses, Molecular Structures, and Countercation‐Induced Structural Transformation of Monomeric α‐Keggin‐Type Polyoxotungstate‐Coordinated Mono‐ and Dipalladium(II) Complexes

AbstractMonomeric α‐Keggin‐type polyoxotungstate‐coordinated mono‐ and di‐palladium(II) complexes, i. e., Cs5[α‐PW11O39{PdII(Me2ppz)}] ⋅ 5H2O (Cs‐1) (Me2ppz=N,N’‐dimethylpiperazine), [(CH3)4N]4H[α‐PW11O39{PdII(Me2ppz)}] ⋅ 8H2O (TMA‐1), and Cs2.5H0.5[α‐PW11O39{PdII(en)}2] ⋅ 7H2O (en=ethylenediamine) (Cs‐2), were synthesized by 1 : 1 or 1 : 2 stoichiometric reactions of mono‐lacunary α‐Keggin polyoxotungstate [α‐PW11O39]7− with PdCl2(Me2ppz) and PdCl2(en) in aqueous solution. Subsequent purification from water afforded these compounds as analytically pure homogeneous yellow precipitates. Single‐crystal X‐ray structural analysis of Cs‐1 revealed that a palladium(II) moiety, i. e., [Pd(Me2ppz)]2+, was coordinated to two oxygen atoms in the mono‐vacant site of [PW11O39]7− with an asymmetric configuration, resulting in an overall C1 symmetry. With regard to the crystal structure of Cs‐2, two palladium species of the [Pd(en)]2+ type were coordinated to two oxygen atoms in the mono‐vacant site with symmetric configuration, resulting in an overall Cs symmetry. A unique structural transformation of the mono‐lacunary α‐Keggin structure of Cs‐1 to the di‐lacunary γ‐Keggin polyoxotungstate Cs7[γ‐PW10O36] was observed upon the addition of cesium chloride.

Evaluating the electronic properties of ditopic and hetero-ditopic ligands derived from benzimidazole and pyrazole by 13C NMR spectroscopy

Huynh’s electronic parameter (HEP) was applied to distinguish the electronic properties of benzimidazole and pyrazole donors in N^N ditopic and N^N′ hetero-ditopic, dinucleating ligands by 13C NMR spectroscopy of their dipalladium complexes bearing terminal iPr2-bimy reporter ligands. The 13Ccarbene NMR resonances of the iPr2-bimy ligand (HEPs) indicate stronger donation of the symmetrical dibenzimidazole compared to that of the dipyrazole with a ΔHEP value of 1.54 ppm. Based on this benchmark value, the effect of spacer groups on the electron donation of the mixed benzimidazole-pyrazole ligands was investigated for the first time. The donicity gap between the benzimidazole and pyrazole donors increases with alkylene spacers, but shrinks with the 1,4-phenylene linker, suggesting electronic communication between the two different heterocycles via π-electron conjugation. Furthermore, these complex probes could also be used to catalyze the direct arylation of pentafluorobenzene, which showed that the unsymmetrically bridged, dinuclear complexes with alkylene linkers are more reactive.

Syntheses, Reactivities, Characterization, and Crystal Structures of Dipalladium Complexes Containing the 1,3-pyrimidinyl Ligand: Structures of [Pd(PPh3)(Br)]2(μ,η2-C4H3N2)2, [Pd(Br)]2(μ,η2-Hdppa)2, and [{Pd(PPh3)(CH3CN)}2(μ,η2-C4H3N2)2][BF4

In a refluxing chloroform solution, the η1-pyrimidinyl {pyrimidinyl = C4H3N2} palladium complex [Pd(PPh3)2(η1-C4H3N2)(Br)], 1 exhibited intermolecular displacement of two triphenylphosphine ligands to form the doubly bridged η2-pyrimidinyl Dipalladium complex [Pd(PPh3)(Br)]2(μ,η2-C4H3N2)2, 3. The treatment of 1 with Hdppa {Hdppa = N,N-bisdiphenyl phosphinoamine} in refluxing dichloromethane yielded the doubly bridged Hdppa dipalladium complex [Pd(Br)]2(μ,η2-Hdppa)2, 4. Complex 1 reacted with the bidentate ligand, NH4S2CNC4H8 and, NaS2COEt, and the tridentate ligand, KTp {Tp = tris(pyrazoyl-1-yl)borate}, to form the η2-dithio η1-pyrimidinyl complex [Pd(PPh3)(η1-C4H3N2)(η2-SS)], (5: SS = S2CNC4H8; 6: SS = S2COEt) and η2-Tp η1-pyrimidinyl complex [Pd(PPh3)(η1-C4H3N2)(η2-Tp)], 7, respectively. Treatment of 1 with AgBF4 in acetonitrile at room temperature resulted in the formation of the doubly bridged η2-pyrimidinyl dipalladium complex [{Pd(PPh3)(CH3CN)}2(μ,η2-C4H3N2)2][BF4]2, 8. All of the complexes were identified using spectroscopic methods, and complexes 3, 4, and 8 were determined using single-crystal X-ray diffraction analyses.

Di-Palladium Complexes are Active Catalysts for Mono-N-Protected Amino Acid-Accelerated Enantioselective C–H Functionalization

The role of mono-protected amino acid (MPAA) ligands in accelerating enantioselective cyclopalladation and palladium-catalyzed C–H functionalization was investigated using kinetic, spectroscopic, a...

Dipalladium(I) complexes of ortho- and para-functionalized 1,3-bis(aryl)triazenide ligands: Synthesis, structure and catalytic activity

The synthesis and characterization of a series dipalladium(I) complexes of formulae [Pd{1-[2′-(methoxycarbonyl)phenyl]-3-[4′-X-phenyl]triazenide}]2 [X = F (4), Cl (5), Br (6)] are reported. The crystal structure of complex 4 was determined by X-ray diffraction studies. The previously reported dipalladium(I) 1–3 as well as the new complexes 4–6 were used as catalysts in the Heck and Suzuki couplings of different p-substituted bromobenzenes.

Characterizing Hydrogen-Bond Interactions in Pyrazinetetracarboxamide Complexes: Insights from Experimental and Quantum Topological Analyses.

Experimental and topological analyses of dipalladium(II) complexes with pyrazinetetracarboxamide ligands containing tetraethyl (1), tetrahexyl (2), and tetrakis(2-hydroxyethyl) ethyl ether (3) are described. The presence of two very short O---O distances between adjacent amide carbonyl groups in the pincer complexes revealed two protons, which necessitated two additional anions to satisfy charge requirements. The results of the crystal structures indicate carbonyl O---O separations approaching that of low barrier hydrogen bonds, ranging from 2.413(5) to 2.430(3) Å. Solution studies and quantum topological analyses, the latter including electron localization function, noncovalent interaction, and Bader's quantum theory of atoms in molecules, were carried out to probe the nature of the short hydrogen bonds and the influence of the ligand environment on their strength. Findings indicated that the ligand field, and, in particular, the counterion at the fourth coordination site, may play a subtle role in determining the degree of covalent association of the bridging protons with one or the other carbonyl groups.

Double-Decker-Type Dipalladium Catalysts for Copolymerization of Ethylene with Acrylic Anhydride

Dipalladium complex with a macrocyclic ligand having two diimine sites catalyzes copolymerization of ethylene with acrylic anhydride to afford the copolymer containing the repeating unit from acrylic anhydride. The obtained copolymer contains the cyclic and acyclic anhydride groups in high content (up to 5.7 mol %) in the main chain. It is contrasted with that the monopalladium complex with the diimine ligand catalyzes the reaction to yield the copolymer having the polar group at the end of branches with lower content (0.8 mol %). Decreased ethylene pressure of the reaction using the dipalladium catalyst results in increase in the polar comonomer unit content. Methanolysis of the copolymer takes place smoothly to give the polymer with both carboxylic acid and ester groups in the polymer chain.

Synthesis and Catalytic Activity of Chiral Linker-Bridged Bis-N-Heterocyclic Carbene Dipalladium Complexes

A series of new chiral-bridged N-heterocyclic carbene ligands with different substituents and their corresponding palladium complexes have been synthesised. The effect of the substituents on the catalytic activity of these Pd complexes was investigated in the Suzuki reaction of p-bromotoluene with phenylboronic acid, and the results showed that the complexes with aryl substituents performed better than those with alkyl substituents. The complex with the most sterically hindered substituent (diisopropyl on a phenyl group) performed best, and it was also an efficient catalyst for the reaction of various arylboronic acids with aryl halides having different electronic and steric properties. In addition, it was employed as a catalyst in the asymmetric Suzuki reaction, but only moderate yields of 1,1-binaphthalenes with less than 20% enantiomeric excess were obtained.

A rare P, C−bridged bimetallic palladium complex

Reaction of (cod)PdCl2 with two molar equivalents of trimethylphosphine (P(CH3)3) followed by equivalent mole of [Li2L], [L=C(Ph2P=NSi(CH3)3)2]2- (1) in THF at ambient temperature, did not yield the anticipated methanediide-Pd complex. However, a C-H activation of one methyl group of one P(CH3)3 ligand on each of two Pd atoms occurred to form, ultimately, a rare example of an unusual P,C-bridged bimetallic palladium complex [{(H3C)3P)PdCl}2(μ:κ1,κ1 – ((H3C)2PCH2)2] (2). The cod was displaced initially and the completely protonated form of the ligand, H2L, and LiCl salt were all generated byproducts of the reaction with the methanediide. The dipalladium complex (2) was fully characterized including solution NMR spectra and a solid state X-ray crystal structure determination. The character of forming metal-ligand bonds in the complex was evaluated with Gaussian03 DFT calculations and the NBO analysis is also presented. Application of NMR Spin-Work simulation and DFT calculation data, the complicated 31P NMR spectrum pattern and P-P spin coupling constants are fully obtained and assigned.

Synthesis and Catalytic Activity of Chiral Dicarbene Dipalladium Complexes Incorporating the S-binaphthol Unit

A series of chiral di-N-heterocyclic carbene (NHC) dipalladium complexes, [{PdPyCl2}2(di-NHC)], in which di-NHC represents a di-imidazolylidene, featuring an (S)-3,3′-dimethyl-2,2′-dimethoxy-1,1′-binaphthalene spacer between the carbene units, have been prepared. The influence of ligand size on the catalytic activity of these complexes in the Suzuki reaction of phenylboronic acid with p-bromotoluene has been investigated. The most sterically hindered complex, bearing the di-isopropylphenyl group, showed the greatest catalytic activity, and it is active for various aryl halides with different electronic and steric properties.

A palladacyclic N-heterocyclic carbene system used to probe the donating abilities of monoanionic chelators.

A series of NHC-containing [C^N]- or [C^C']-type palladacyclic complexes of the general formula [PdBr(iPr2-bimy)(L^X)] (5-8, 11, 12, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been synthesized and fully characterized. Using these complexes, the donating abilities of monoanionic chelators were probed for the first time. The [C^N]-type palladacycles 5-8 were prepared from acetato-bridged dipalladium complexes [Pd(μ-CH3COO)(C^N)]2 (1-4) and iPr2-bimy·H+Br-as precursors. In the case of the [C^C']-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr2(iPr2-bimy)(trz)] (8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the iPr2-bimy probe were first prepared followed by acetate-assisted cyclopalladations. The 13Ccarbene NMR signals of the iPr2-bimy ligands in all complexes (i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or [L^X]-type chelators with the exception of the Bzpy ligand (Bzpy = 2-(2-pyridinylmethyl)phenyl-C,N). This has been attributed to its larger bite angle, the resulting varied coordination geometry and the lack of electronic delocalization between the two donor units. The donicities of [L^X]-type chelators studied in this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.

Palladium(II) Complexes of Homologated Ferrocene Phosphanylether and Thioether Ligands

The reaction of [1′‐(diphenylphosphanyl)ferrocenyl]methanol/borane (1:1) with in situ formed N,N,N′,N′,S‐pentamethylisothiouronium iodide and sodium hydride, followed by removal of the borane protecting group with 1,4‐diazabicyclo[2.2.2]octane, afforded 1‐(diphenylphosphanyl)‐1′‐[(methylthio)methyl]ferrocene (3) as a new, homologated, hybrid phosphanylferrocene ligand. Compound 3 and the congeneric phosphanyl ether 2 were studied as ligands in PdII complexes. When treated with [PdCl2(MeCN)2] in a Pd/ligand ratio of 1:1, compound 3 furnished a mixture of two Pd complexes, including the ligand‐bridged dimer [{µ(P,S)‐3}PdCl2]2 (6), which was structurally characterized. Upon increasing the amount of ligand to 2 equiv., a similar reaction produced the bis(phosphane) complex [PdCl2(3‐κP)2] (7). Bridge‐cleavage reactions of the dipalladium complex [(LNC)Pd(µ‐Cl)]2 {LNC = 2‐[(dimethylamino‐κN)methyl]phenyl‐κC1} with donors 3 and 2 proceeded in a uniform manner to give the corresponding phosphane complexes [(LNC)PdCl(3‐κP)] (8) and [(LNC)PdCl(2‐κP)] (9). Conversely, removing the Pd‐bonded chloride from these complexes with AgClO4 generated the bis(chelate) complex [(LNC)Pd(3‐κ2P,S)] (10) and the aqua complex [(LNC)Pd(H2O)(2‐κP)] (11), respectively, both of which could be converted back into their precursors by adding Bu4NCl. The structures of the complexes 6–11 (some in solvated form) were determined by single‐crystal X‐ray diffraction analysis.

Catalytic Reduction of Nitroarenes by Dipalladium Complexes: Synergistic Effect

The direct reaction between 2,7-bis(2-pyridinyl)-1,8-naphthyridine (bpnp) and Pd(CH3COO)2 in CF3COOH yields the new dinuclear palladium(II) complex [Pd2(bpnp)(μ-OH)(CF3CO2)2](CF3CO2) (1). Similarly, substitution of Pd(CH3CN)4(BF4)2 with bpnp in DMF gives [Pd2(bpnp)(μ-OH)(DMF)2](BF4)3 (2). Treatment of 1 or 2 with Cl– readily provide the chloro-substituted species [Pd(bpnp)(μ-OH)(Cl)2]+. All complexes were characterized by spectroscopic methods, and the structure of 2 was further confirmed by X-ray crystallography. Complex 1 is an efficient catalyst for the reduction of aromatic nitro compounds leading to the corresponding aniline derivatives under atmospheric pressure of hydrogen at 50 °C. The mechanistic pathway of the catalysis is investigated. From the reaction pathway, it is suggested that a facile condensation of nitroso and hydroxylamine intermediates is enabled by the dipalladium system and the desired transformation proceeds smoothly under mild reaction conditions to yield the reduced product.

Hydrogenation of nitroarenes catalyzed by a dipalladium complex

A dipalladium complex [Pd2(L)Cl2](PF6)2 (2), via the substitution of (PhCN)2PdCl2 with 5‐phenyl‐2,8‐bis(6′‐bipyridinyl)‐1,9,10‐anthyridine (L) followed by the anion exchange, was found to be a good pre‐catalyst for the reduction of nitroarenes to yield the corresponding anilines under atmospheric pressure of hydrogen in methanol. This method provides a straightforward access to a diverse array of functionalized anilines, exhibiting a possible application in synthetic chemistry. The catalytic activity of this complex is enhanced by the di‐metallic system via the synergistic effect.

Conjugates of natural chlorins with cyclen as chelators of transition metals

Novel herein synthesized conjugate of chlorin e 6 methyl ester with cyclen, as well as its mono- and dipalladium complexes, are regarded in view of their use in therapy and diagnostics.

Synthesis, structure and catalytic activity of xylylene-bridged dipalladium complexes with triazolylidene ligands

A series of di-N-heterocyclic carbene (NHC) dipalladium complexes, [PdPyBr2]2(di-NHC) (2a–h), in which di-NHC represents a di-triazolylidene, featuring a 1,4-xylylene spacer between the carbene units, have been prepared from the reactions of the corresponding ditriazolium bromides with PdCl2 and excess NaBr as an additive in the presence of K2CO3 in pyridine. All of the complexes were fully characterized by NMR spectra, IR and elemental analyses. The molecular structure of 2b was determined by X-ray diffraction study, showing the intermolecular hydrogen bonding of Br···H–C. The influences of size of ligands on the catalytic activity have been investigated in the Heck reaction of styrene with bromobenzene. The complex 2g showed the best catalytic activity, and it is active for various aryl bromides with different electronic and steric properties and styrenes with both electron-donating and electron-withdrawing groups.

Synthesis of Xylylene-Bridged Dipalladium Complexes with Imidazole and Triazole-Based Di-N-Heterocyclic Carbene (NHC) Ligands

The di-N-heterocyclic carbene (NHC) dipalladium complexes, [(PdPyBr2)2(di-NHCimi)] and [(PdPyBr2)2(di-NHCtri)], di-NHCimi and di-NHCtri represent an imidazolylidene and a triazolylidene, respectively, featuring a 1,4-xylylene spacer between the carbene units, have been prepared. The influences of the imidazolylidene and triazolidene backbone and different substituents on the catalytic activity have been investigated in the Mizoroki–Heck reaction of styrene with bromobenzene.